An inductive power device, such as a power transformer, used by public utilities, typically has its power windings immersed in a fluid coolant, usually oil, within a transformer tank. During operation the windings and a core, around which the windings are wound, become heated and thereby heat the surrounding fluid. In general, the fluid is in turn cooled by a number of cooling devices provided outside of, but in connection to, the transformer. The cooling devices are typically provided with different fans to cool a flow of oil from the transformer. Upon heating, the fluid rises upwards to the top of the tank containing the fluid. Heated fractions of the fluid, extracted from the top of the tank is then cooled by the cooling devices and returned at the bottom of the transformer tank. Consequently, a cooling of the fluid and thereby indirectly of the power windings and core is accomplished.
A monitoring of the condition of an inductive power device is of benefit for a reliable and efficient operation. In particular, for a transformer the temperatures of the windings and of the cooling fluid need to be tightly followed. Serious accidents, possibly leading to the destruction of the entire or part of the transformer or a shortening of its life, may happen if the temperature of the windings and/or fluid increases over certain limits. Therefore, monitoring of the temperature of the fluid and/or windings is a common security measure when operating power transformers.
Several methods for obtaining the temperature of the windings and/or fluid exist in the prior art, being based on either simulations, calculations or more direct measurements. In its simplest form, a simulation of the winding temperature is performed by biasing a thermometer reading by an amount proportional to the winding current. A heater coil, or similar means, is normally arranged in connection to a thermometer in the fluid. The heater coil is in turn controlled by a current transformer, which provides the heater coil with a current proportional to the winding current.
In the international patent application WO 99/60682 a method for determining temperatures, among others average and hot-spot temperature, of an oil-cooled transformer is disclosed. In this application, hot-spot temperatures are obtained without any direct temperature measurements. Instead, transformer terminal voltage, winding current and ambient temperature are measured, and status of ventilators and pumps and position of a step switch are determined. These variables are then fed into a thermohydraulic model, whereby the temperatures are calculated based on transformer losses, heat transfer convection parameters, flow resistance and oil flow. These calculated temperatures may then be used for controlling and operating the power transformer, in order to avoid overheating.
In the two patent U.S. Pat. No. 4,745,571 and U.S. Pat. No. 4,775,245, methods and systems for simulating and controlling the temperature of a fluid-cooled power transformer are disclosed. The methods electronically compute the transformer winding temperature based on a measured top temperature of the fluid and an incremental additional temperature resulting from transformer load currents. These two temperatures are then added and used for controlling indicators, cooling fans and circuit trips. Consequently, the temperature is monitored for controlling the operation of the transformer, in that no temperature damages should occur.
The disclosures discussed above all concern the protection against acute damage of an inductive power device. A too high temperature will according to these disclosures lead to actions of load reduction and/or increased cooling efforts. However, a malfunctioning inductive power device may not necessarily lead to a dangerous heating at all instances. A malfunctioning inductive power device may e.g. operate at a low fraction of its rated capacity and will then despite its defects operate at a permitted temperature. No indication of the defect is detected until the power is increased. At such an occasion, it is often very inconvenient to take the inductive power device out of operation for repair, since it typically coincides with an increased performance demand.
It would therefore be advantageously if one, as a complement to the conventional overtemperature protection devices, could achieve an apparatus for providing information about the actual operation condition, related to heat issues, of the inductive power device at any instant, i.e. a condition diagnosing apparatus.